This invention relates to a radioactive logging technique and more particularly to a prompt fission neutron uranium assaying technique.
When a formation containing a uranium ore is irradiated with fast neutrons, the uranium nuclei react to neutron bombardment by breaking into smaller nuclear fractions which are normally referred to as fission products. The fission of uranium is attended by the emission of prompt neutrons immediately upon occurrence of the fission reaction and also by the emission of delayed neutrons by the fission products subsequent to the fission reaction. The prompt fission neutrons are emitted at the time of the fission reaction, while the delayed neutrons are emitted by the fission products for an appreciable length of time following the fission reaction.
The use of fast neutron irradiation for the detection of uranium has also been explored in a paper by Jan A. Czubek, "Pulsed Neutron Method for Uranium Well Logging," GEOPHYSICS, Vol. 37, No. 1, Feb. 1972, pp. 160-173. Czubek examines several phenomena associated with fast neutron irradiation of uranium-bearing formations and concludes that three can be employed to advantage in uranium detection techniques. Those which Czubek proposes for use as uranium content indicators are (1) epithermal neutron intensity resulting from prompt thermal neutron fission of uranium 235, (2) delayed thermal neutron intensity from prompt thermal neutron fission of uranium 235, and (3) delayed thermal neutron intensity from fast neutron fission of uranium 238. The author also sets forth a number of relationships including Equations 12 and 31 as set forth below: ##EQU1## wherein, t.sub.2 is the end of the measurement period,
t.sub.1 is the start of measurement period after the beginning of the neutron burst, PA1 Q is the average neutron output, PA1 t.sub.T is the total measurement time, PA1 .DELTA.T is the time width of neutron burst, PA1 .rho. is bulk density, PA1 .SIGMA..sub.D is the macroscopic absorption cross section for neutron detector, PA1 v.sub.Cd is the neutron velocity for cadmium cutoff, PA1 t.sub.s is the slowing-down time to cadmium cutoff energy, PA1 .upsilon. is the number of secondary neutrons per fission, PA1 .tau. is the mean lifetime of thermal neutrons in the medium, PA1 P.sub.U is the percent weight content of uranium in the ore, PA1 .sigma..sub.f is the thermal fission cross section of .sup.235 U, PA1 .alpha..sub.1 is the percent isotopic abundance of .sup.235 U, PA1 N.sub.o is Avogadro's number, PA1 A.sub.235 is the atomic mass of .sup.235 U, PA1 A.sub.238 is the atomic mass of .sup.238 U, PA1 v is the thermal neutron velocity, PA1 .SIGMA..sub.a is the macroscopic absorption cross section of the medium for thermal neutrons, PA1 .DELTA.t is t.sub.2 -t.sub.1, PA1 T is the time between successive neutron bursts, PA1 .epsilon..sub.Di is the number of delayed neutrons per fission for the ith group of delayed neutrons, PA1 .lambda..sub.i is the decay constant for the ith group of delayed fission neutrons, PA1 .sigma..sub.ff is the fast fission cross section for .sup.238 U, PA1 .alpha..sub.2 is the isotopic abundance of .sup.238 U (in percent), and PA1 .SIGMA..sub.fa is the macroscopic absorption cross section for fast neutrons.
Equation 12 establishes the relationship between neutron count and uranium ore grade for epithermal neutron detection and Equation 31 establishes the relationship between neutron count and uranium ore grade for detection of delayed thermal neutrons resulting from fast neutron fission of uranium 238 and thermal neutron fission of uranium 235.
Among the rock parameters presented as independent variables in one or both of these equations are bulk density .rho. (Equations 12 and 31), the macroscopic absorption cross section for thermal neutrons .SIGMA..sub.a (Equations 12 and 31), and the slowing-down time of neutrons t.sub.s (Equation 12). Czubek on pages 172 and 173 discusses the necessity for making compensatory measurements for bulk density, slowing-down time, and the thermal neutron mean life, also referred to as neutron lifetime .tau.. He states that the bulk density can be determined by gamma-gamma density logging, the slowing-down time for porosity measurements obtained by conventional neutron logging or by employing the neutron generator, and the mean life (or lifetime) by pulsed neutron logging.
In U.S. Pat. No. 3,686,503 to Givens et al, there is disclosed a borehole logging system for characterizing the uranium content of natural earth formations on the basis of measurements of delayed neutrons resulting from neutron fission of uranium. This patent discloses a subsurface assaying operation which is carried out by locating in a borehole adjacent a formation of interest a logging tool which includes a source of fast neutrons and a thermal neutron detector. The formation is irradiated with repetitive bursts of fast neutrons; and subsequent to each burst and after dissipation of the original source neutrons, delayed neutrons resulting from neutron fission of uranium are detected. The output from the detector is then recorded in order to obtain a log indicative of the uranium content of the formation.